Aluminum-magnesium alloy



Patented Nov. 21, 1933 UNITED STATES PATENT OFFICE ALUMINUM-MAGNE SIUMALLOY Robert S. Archer, Lakewood, Ohio, Company of America,

Aluminum assignor to I Pittsburgh,

Pa., a corporation of Pennsylvania No Drawing. Application May 21, 1930Serial N0."454,509

6 Claims. (01. 75-1) This invention relates to aluminum-magnesium alloyscontaining substantial amounts of magnesium, say 5 per cent or'more, upto about per cent. Such alloys have certain advanta- 5 geous propertiesbut they also possess certain 5 advantage of low cost is offset 'or morethan offset by lack of resistance to corrosive agencies. It is thereforethe chief object of my present invention to provide aluminum-magnesiumalloys in which the aluminum may be no better than commercial grade asregards totalimpurity content but which shall nevertheless exhibit adegree of corrosion resistance comparable to that heretofore obtainableonly in alloys made with the expensive high-purity metal referred to. By

5 "aluminum of commercial grade or f commercial aluminum I mean suchaluminum as the ordinary ingot metal of commerce, which, as is wellknown, averages not more than about 99.2 or

99.3 per cent pure, the remainder being for the most part silicon andiron, sometimes with small amounts of copper.

I have found that of the impurities 0511011 present in amountssuflicient to have an important eifectupon the corrosion resistance ofalu- 5 minum alloys of high magnesium content, say 5 per cent or more,iron is the mostdetrimental; and I have determined further, by testsmade with an aqueous solution containing .20 per cent of sodium chloridand 0.3 per cent of hydrogen 1 peroxid, that the iron impurity should.be less,

greater, than thesilicon imable, by using aluminum of or in any case notpurity. I am thus low iron impurity,- though relatively high in sili- Icon impurity, to provide aluminum-magnesium alloys possessing corrosionresistance adequate for uses in which resistance to corrosive agenciesis of prime importance.

In the tests referred to above the concentration of hydrogen peroxid wasmaintained substantially constant and the time of immersion of thetestspecimens .was forty-eight hours. The corrosion resistance wasjudged visually by ex amination with the naked eye and with themicroscope, and quantitativelyby weight of the ig- H nited corrosionproduct. Taking as a standard an alloy containing 10 'cent silicon and1.01 per cent iron the corrosion was 695 per cent greater than thestandard.

Typical results, found with alloys containing 10 to 10.68 per cent ofmagnesium, are tabulated as follows, the corrosion of the standard(designated 'A) being taken as unity for purposes of comparison!Table 1. Low. silicon, increasing iron All Percent Percent PercentPercent Corrooy Cu Si Fe S1+Fe sion Table 2.-Inc1'easing silicon, iron0.05%

An Percent Percent Percent Percent' Corro- 4 Cu Fe s1+r 5100 0. 04 0. 100. 02 0. 12 1. 00 0.05 0.25 0.05 0. so 1.10 o. 05 0. 57 0. 05 o. 02 1.21 0.05 0.00 0.05 1.04 1.13

Table 3. ncreasing silicon," iron 0.60i0.05%

. Percent Percent Percent Percent Corro- Y Cu s1 r5 s1+r0 S1011 0. 04 0.10 0. 02 0.12 1. 00 0. 05 0. 07 0. 50 0. 0a 0. 10 0. 00 0. 2s 0. 05 0.885. 00 0.00 0 34 0.04 0. 0a 5.28

Table 4.Decreaszng silicon, increasing zron 1 An Percent Percent PercentPercent Corro- Cu 211 Fe Si+Fe 51011 Adding 0.05 per cent of copper tothe 0.33 and 0.30 per cent of silicon-plus-iron of alloys B and Erespectively, we have 0.38 and 0.35 per cent of copperandsilicon-plus-iron, making these alloys considerably better than would beconsidered commercialmetal. Nevertheless alloy B, which contained threetimes as much iron as silicon, suffered more thP-n three times as muchcorrosion as did the standard alloy A, the actual-increase being3.351.00=235 per cent of 1.00; whereas alloy E, containing only 0.03 percent less silicon-plus-iron but having five times as much silicon'asiron, showed'only 19 per cent (1.19- 1.00:19 per cent of 1.00) morecorrosion than the standard. In alloys C and F (each containing 0.05 percent of copper) the diiference in their content of silicon-plus-iron isonly 0.01 per cent; but alloy C, containing 0.56 per cent of iron andonly 0.07 per cent silicon, showed a corrosion six times that of thestandard, an increase of 500 per cent over the latter, whereas alloy F,containing (in addition to 0.05 percent copper) 0.05 per cent of ironand 0.57 per cent silicon; showed only 21 per cent greater corrosionthan the standard. Similarly in alloy D, containing 0.05 per centcopper, 0.10 per cent silicon and 1.01 per cent iron, the corrosion wasnearly 600 per cent more than in the standardalloy, while in alloy G,containing 0.05 per cent copper, 0.99 per cent silicon and 0.05 per centiron, the corrosion.

was only 13 per cent more than the standard.

The efiect of varying proportions of silicon and iron is indicated Table4. Alloys G and. D differ in silicon-plus-iron content by only 0.07 percent; and yet alloy'D, in which the silicon to iron ratio is 1:10, hasmore than seven times the corrosion of alloy G in which the silicon toiron ratio is approximately 20:1, more than 6.5 times the corrosion ofalloy F in which the ratio is 11:1, and more than three times thecorrosion of alloy 'J in which the ratio is less than 1:1. Alloys G andD, approximately 98.9 and 98.8 per cent pure, would be considered aboutequal in grade, yet as regards corrosion resistance Dis very bad while Gis practically as good as the highly pure alloy A.

The eflect of iron described above commercial aluminum-magnesium alloyshas been found to hold over a range of magnesium content from about 5per cent to about 16 per cent and in some cases more, the corrosion lossincreasing as the iron content is increased, or, alternatively, thecorrosion resistance increasing as the iron content is decreased, inrespect to the silicon content. I am therefore able, using aluminum ofrelatively low purity, as for example 99.0 pure, which is lower or atany rate no higher than the average of present commercial ingotaluminum, to provide aluminum-magnesium alloys having corrosionresistance comparable to that of alloys made with aluminum of highpurity, say 99.85

per cent pure. The magnesium may be. of ordinary commercial grade (say99.0 per cent pure) or better, since the amount of iron ordinarily roundin magnesium is very small at most; and for the further reason that thetotal amount of iron imported into the alloy by the magnesium is smallbecause the magnesium is a minor ingredient in comparison with thealuminum. In short, with commercial magnesium the amount of iron carriedinto the .alloy by the magnesium is so slight as to be negligible.

Difierent lots of commercial aluminum may vary considerable not only intotal impurity content but also in amount of specific impurities. Forinstance one lot may have more iron than will afford the degree ofcorrosion resistance desired for a particular purpose and may thereforebe unusable, while another lot may have less iron than necessary. Insuch cases the first lot may often be used'by mixing the two. In somecases the iron content may be brought down to a usable amount by mixingmore or less highly pure aluminum with the high iron metal. 0r bothexpedients may be employed. Thus by proper selection and combination ofcheaper grades of aluminum, sometimes with admixture of a relativelysmall amount of pure metal, it is possibleto produce an alloy ofadequate corrosion resistance at lower cost than would be entailed bythe use of all high-purity metal.

I have also found it possible with a given iron content, to addcalculated amounts of silicon'so as to obtain a silicon-iron ratiodesirable for favorable corrosion resistance characteristics.

I claim 1. Corrosion resistant aluminum magnesium alloy consisting ofaluminum and about 5 to 16 per cent of magnesium, the alloy being ofcommercial grade as evidenced by a silicon-plus-iron content of fromabout 0.3 to 1 per cent; the ratio of silicon to iron being not lessthan about 1:1.

2. Corrosionresistant aluminum magnesium alloy consisting of aluminumand about 5 to 16 per cent of magnesium, the alloy being of commercialgrade as evidenced by a-content of silicon and iron from about 0.6 to 1per cent and the iron being from about 0.05 to 0.35 per cent; the ratioof silicon to iron being not less' than about 1:1. h

3. Corrosion resistant aluminum 4 magnesium alloy consisting of about 5to 16 per cent of mag,- nesium and the balance aluminum, made ofcommercial aluminum and magnesium, as evidenced by a content of siliconand iron in total amount from about 0.6 to 1 per cent; the ratio ofsilcon to iron being not less than about 1:1.

4. Corrosion resistant aluminum magnesium alloy consisting of aluminumand about 10 per cent of magnesium, containing impurity siliconplus-ironfrom about 0.3 to 1 per cent; the silicon to iron ratio being not-lessthan about 1:1.

5. Corrosion resistant aluminum magnesium alloy consisting of aluminumand about 10 per cent of magnesium, together with silicon and iron intotal amount from about 0.3 to 1 per cent; the iron being not more thanabout 0.1. per cent.

6. Corrosion resistant aluminum -'magensium alloy consisting of aluminumand about 10 per cent of magnesium, together with 0.6.to 1 per cent ofsilicon-plus-iron', the ratio of silcon to iron being greater than 1:1.

